The invention relates to a method for the adjustment of a semiconductor disc relative to a radiation mask in x-ray photolithography as specified in detail in the preamble of main claim 1.
In the semiconductor technolgy photolithographic processes are applied in the fabrication of semiconductor components and integrated circuits where structures of a radiation mask are transmitted to a radiation-sensitive lacquer layer on a semiconductor disc. In the entire fabrication process it is necessary to cause a plurality of layers of structures to be matched. That is why in the different exposure phases the semiconductor disc must be adjusted very precisely against the prevailing mask. Thereby the error in the adjustment of the masks relative to the semiconductor discs must be smaller than the smallest structure to be produced in the semiconductor disc. Under the present fabrication procedure for integrated circuits the transmission of the structure in most cases is accomplished by light optical methods. The adjustment is carried out by the simultaneous observation of two pairs of adjustment marks, with the one pair of adjustment marks being located on the mask and the other one of the semiconductor disc. However, for very precise adjustments in the area of 0.5 micron, the borderline of the resolving power of the ordinary light-optical microscopes is reached.
In x-ray photolithography a varnish sensitive to x-rays and x-ray radiation for the picturing of the mask is used. Based on the short wavelength the phenomena of diffraction are reduced in this type of projection, so that the structural dimensions which can be projected are substantially smaller than in case of projection with visible light. That is why with x-ray photolithography an adjustment with a precision of about 0.1 micron must be attained.
Under the conventional adjustment with an optical microscope first the mask and the semiconductor disc are pressed on each other in order to align the semiconductor disc parallel with the mask, the so-called compensation of the wedging error. Due to surface roughness of the semiconductor disc the mask can thereby be destroyed easily.
With the actual adjustment process, depending on the definition in depth, a small distance is maintained between mask and semiconductor disc. Due to surface roughnesses of the semiconductor disc, however, there the mask again can be destroyed very easily. This is an important reason for a poor yield in the fabrication of semiconductors.
Additional difficulties can occur in an adjustment system used for x-ray photolithography, in that the material of the support on which the structures of the exposure mask to be adjusted either are opaque or only very weakly translucent, so that in view of this fact a simultaneous sharp focusing of the adjustment microscope on the adjustment mark of the mask and on the adjustment marks of the semiconductor disc is complicated.
An adjustment method for x-ray photolithography is known from Solid State Technology (July, 1972), pages 21-26, where the adjustment is made with the aid of x-rays. For that purpose adjustment marks of material absorbing x-rays are applied in each case to the semiconductor disc and to the radiation mask. After the x-rays have penetrated these adjustment marks, the intensity of the radiation is read with the aid of a detector and depending on the shaping of both adjustment marks, an adjustment is effected to provide a radiation maximum or radiation minimum in the detector.
In this method known from prior art it is disadvantageous that no possibility exists for a control of the mutual spacing of semiconductor disc and adjustment mark. For a parallel adjustment of semiconductor disc and radiation mask, this method too requires semiconductor disc and radiation mask to be pressed against each other.